THE MECHANICAL CHANGES OF MUSCLE 



203 





THE EXTENSIBILITY OF MUSCLE 



Living muscle in a perfectly normal condition is distinguished by its slight but 

 ^ perfect elasticity ; that is to say, it is con- 

 siderably stretched by a slight force (in the 

 longitudinal direction), but returns to its 

 original length when the extending weight 

 is removed. The length to which muscle is 

 stretched is not proportional to the weight 

 used, but any given increment of weight 



gives rise to less elongation the more the 

 FIG. 62. Extensibility of india-rubber (a) mugcle ig alread stretched> The accom _ 

 compared with that of a irog s gastroc- ... 



nemius muscle (6). panying curves show diagrammatically the 



elongation of muscle as compared with a 

 piece of india-rubber when the weight on it is uniformly increased. 



Dead muscle is less extensible and its elasticity is less perfect. A given weight 

 applied to a dead muscle will not stretch it so much as when the muscle was alive, but 

 the dead muscle does not return to its original length when the weight is removed. 

 A contracted muscle, on the other hand, is more extensible than a muscle at rest. 

 A gramme applied to a contracted gastrocnemius will cause greater lengthening than 

 if it were applied to the 

 same muscle at rest. The 

 relation between the exci- 

 tability of a muscle under 

 the two conditions of 

 contraction and rest are 





shown in the diagram in 

 Fig. 63. 



At the point y the 

 muscle is unable to shor- 

 ten at all against a 

 weight. It is evident 

 from this diagram that FIG. 63. Curve showing the length of a muscle under various 

 the height of contraction loads in the contracted condition by, and uncontracted 

 of a iscle dim condition cy. The double lines a, 6, &c., represent the con- 



tracted muscle, while the long single lines a c, &c., show the 

 the load is increased, very length of the inact ive muscle. 

 rapidly if the muscle is 



after-loaded, less rapidly if the weight applied to the muscle be allowed to extend it 

 at rest. It is evident however that in either case the diminution in height is not in 

 proportion to the load and that the work done by the muscle, w X h, as the weight is 

 increased, rises at first quickly, then more slowly to a maximum to sink finally to 

 zero. By inspection of diagram (Fig. 63) it will be seen that 



O.h 



<20.h 2 < 30.h 3 > 40.h 4 > 50.h 5 



so that in this case the maximal mechanical work is obtained when the muscle is loaded 

 with about 30 gms. 



PROPAGATION OF CONTRACTION. THE CONTRACTION WAVE 



The whole muscle does not as a rule contract simultaneously. When 

 excited from its nerve the contraction begins at the end-plates and spreads 

 in both directions through the muscle. The rate of propagation of the con- 

 traction wave can only be measured by employing a curarised muscle, so as 

 to avoid the wide spreading of the excitatory change by means of the intra- 

 muscular nerve-endings. For this purpose a curarised saitorius muscle 



